This performs a few touch-ups to the OSX installer:
* A rust logo is shown during installation
* The installation happens to /usr/local by default (instead of /)
* A new welcome screen is shown that's slightly more relevant
This fixes issue #8731 . On Windows, if the git option core.autocrlf is true (github recommends users to turn on this flag - see https://help.github.com/articles/dealing-with-line-endings), the headers generated from the .in files will have syntatic errors, causing compilation to fail.